My youngest had a similar problem, except he was too tall for his car seat when he was 2 1/2 y/o. So we bought the Graco Nautilus 3 in 1 car seat. My 8 year old can even fit in the Nautilus's 5 point harness & he is over 4 ft. tall!!!
***Please look into the Graco Nautilus 3 in 1 car seat.*** You can find them at Wal-mart & Target. They are about $150, (but that is worth the safety of your child). They have a 5 point harness that holds a child up to 65 lbs., then they turn into a booster seat & go up to 100 lbs.! I think they are the most awesome seats! Easy to use & much less expensive than the other seats that fit larger children.
God bless!
P.S.
It is not safe to move a 2 1/2 y/o into a booster seat, even if the child does weight 40 lb. Boosters are made for larger children (5+ y/o). I have seen 1 too many videos about children who are 3 y/o, put into a booster seat & killed in a car wreck b/c the booster seat with was not made for smaller children.